This application is based on application No. 2000-5732 filed in Japan on Jan. 6, 2000, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an imnge-sensing apparatus, and in particular to an image-sensing apparatus incorporating a solid-state image-sensing device having a characteristic that an output voltage thereof varies natural-logarithmically in accordance with the amount of received light.
2. Description of the Prior Art
FIG. 1 shows a solid-state image-sensing element (hereafter referred to as a xe2x80x9cpixelxe2x80x9d) employed in a conventional solid-state image-sensing device having a characteristic that an output voltage thereof varies natural-logarithmically in accordance with the amount of received light. The pixel shown in FIG. 1 is composed of a photodiode PD that receives at its cathode a voltage Vdd1, an N-channel MOS transistor Tr1 that has its drain and gate connected to the anode of the photodiode PD and that receives at its source a voltage Vss1, an N-channel MOS transistor Tr2 that has its gate connected to the gate of the transistor Tr1 and that receives at its drain a voltage Vdd2, and a capacitor C that has one end connected to the source of the transistor Tr2 and that receives at the other end a voltage Vss2. Here, the voltage appearing at the node between the source of the transistor Tr2 and the capacitor C is used as the output voltage Vout. The voltages mentioned above fulfill the relations Vdd1 greater than Vss1 and Vdd2 greater than Vss2.
How this pixel works will be described briefly below. This pixel exploits the subthreshold characteristics that the transistor Tr1 exhibits when its gate-source voltage is lower than the threshold voltage. When the photodiode PD receives light and, as a result, a current Ip flows through the transistor Tr1, the gate voltage Vg of the transistor Tr1 varies natural-logarithmically in accordance with the current Ip. This gate voltage Vg causes a current to flow also through the transistor Tr2, and thus electric charge is accumulated in the capacitor C. As a result of this electric charge being accumulated in the capacitor C, the output voltage Vout appears, which is given as                     Vout        =                  Vss1          +                                    nkT              q                        ⁢            ln            ⁢                          xe2x80x83                        ⁢                          (                                                q                  nkTC                                ⁢                                  ∫                                      Ip                    ⁢                                          xe2x80x83                                        ⁢                                          ⅆ                      t                                                                                  )                                                          (        1        )            
where q represents the amount of electric charge carried by an electron, k represents the Boltzmann constant, n represents a constant determined according to the structure of the transistor, T represents the absolute temperature, and C represents the capacitance of the capacitor C.
An image-sensing apparatus is equipped with a solid-state image-sensing device that has a plurality of pixels, each having a characteristic as described above, arranged in an array and that is further provided with transfer means for transferring the output voltages from the individual pixels. In such a solid-state image-sensing device, the individual pixels have different sensitivities, which are difficult to make completely even, and therefore, even if uniform light is shone on them, the voltages output from the individual pixels differ from one another. To overcome this problem, U.S. Pat. No. 5,289,286 proposes an image-sensing apparatus in which the differences in sensitivity among the individual pixels are compensated for.
The configuration of the image-sensing apparatus proposed in U.S. Pat. No. 5,289,286 mentioned above will be described briefly below with reference to FIG. 11, which shows a block diagram thereof. The image-sensing apparatus shown in FIG. 11 is composed of an image-sensing device (hereafter referred to as the xe2x80x9csensorxe2x80x9d) 1 that has a plurality of pixels each producing an output voltage varying natural-logarithmically in accordance with the amount of received light and that has transfer means for transferring the outputs from the individual pixels, an analog-to-digital converter (hereafter referred to as the xe2x80x9cA/D converterxe2x80x9d) 2 for performing analog-to-digital conversion on the voltage output from the sensor 1, a memory 3 for storing the digital values (hereinafter referred to as the xe2x80x9cshading dataxe2x80x9d) obtained as a result of conversion performed by the A/D converter 2, a correction calculation circuit (hereafter referred to as the xe2x80x9cCORxe2x80x9d) 4 for subtracting the shading data stored beforehand in the memory 3 from the image data fed from the A/D converter 2 during actual image sensing so as to correct the image data, and a device 5, such as a storage device or printer, that is fed with the image data corrected by the COR 4.
In this image-sensing apparatus configured as described above, first, uniform light is shone on the sensor 1, and, for each pixel, the digital data representing the output voltage obtained therefrom is stored in the memory 3. Then, during actual image sensing, the image data acquired is corrected on the basis of the above-mentioned digital data, called the shading data, that was stored in the memory 3 when uniform light was shone on the sensor 1, and the thus corrected image data is output to the device 5. By correcting the image data in this way, it is possible to correct the unevenness in sensitivity among the individual pixels of the sensor 1.
However, in a conventional image-sensing apparatus configured as shown in FIG. 11, a large amount of data needs to be stored as shading data in the memory 3, because this data is obtained as constituting one full screen from the sensor 1. To permit such shading data, constituting one full screen, to be stored in the memory 3, the memory 3 needs to be given a large capacity. Inconveniently, giving the memory 3 a large capacity hampers miniaturization and cost reduction of the apparatus.
An object of the present invention is to provide an image-sensing apparatus in which shading data is stored after being compressed so as to minimize the memory capacity required for its storage.
To achieve the above object, according to the present invention, an image-sensing apparatus is provided with: a solid-state image-sensing device composed of a plurality of pixels each having a characteristic that that output voltage thereof varies natural-logarithmically in accordance with the amount of light received; a compressor for performing data compression on calibration data used to correct variations in the output voltages of the individual pixels of the solid-state image-sensing device; a memory for storing the calibration data compressed by the compressor; an expander for performing data decompression on the calibration data stored in the memory; and a calibrator for calibrating, in accordance with the calibration data decompressed by the expander, output data representing the output voltages obtained from the individual pixels of the solid-state image-sensing device during image sensing.